Selectivity of Sulfonated Cation-Exchange Resin for Metal Cations

The selectivity of sulfonated ion-exchange resins for metal cations is often expressed quaUtatively in terms of elution orders. Numerical selectivity data for cations are Umited [3-5]. Strelow and coworkers [6-8] pubUshed comprehensive lists of distribution coefficients for metal ions with perchloric acid and other mineral acid eluents. However, their data are for sulfonated gel resins of high exchange capacity. 

The retention factors were calculated from the retention times and are given in Tables 5.3 and 5.4. The data show that, as expected, eluents containing sodium(I) are more efficient than those containing hydrogen]I). The data in these tables are arranged in order of increasing capacity factors. In this way it is possible to compare the relative affinities of the various divalent and trivalent metal ions for resin sites. Note that there are crossovers in the retention factors of lead(II) and several trivalent metal ions as the eluent concentration is increased. 

The elution order of metal ions reported in Table 5.4 is similar to that reported earlier by Strelow and Sondrop [8] with perchloric acid eluent and gel resins of high exchange capacity. Strelow and coworkers noted anion-complexing effects on elution orders in several cases when eluent acids other than perchloric acid were used. 

13) predicts that a plot of k against the activity of the eluent ion should be a straight line. If the eluent ion is monovalent, the slope should be the same as the charge on the metal ion, M. 

The data in Table 5.3 (perchloric acid eluent) and Table 5.4 (sodium perchlorate eluent) were plotted according to Eq. (5.13). Linear plots were obtained in all cases. The slopes, obtained by linear regression, are given in Table 5.5 for perchloric acid, and similar results were obtained for sodium perchlorate. In most cases, the negative slope was very close to the charge on the metal ion. The somewhat 

Sevenich and Fritz [9] published a comprehensive study of metal cation selectivity with resins of a more modern type. The studies were made on a column packed with a 12% cross-linked polystyrene-divinylbenzene resin 12-15 mm in diameter and with an exchange capacity of 6.1 pequiv/g. The resins were prepared by rapid sulfonation so that the sulfonic acid groups are concentrated on the outer perimeter of the resin beads [10]. 

---

Selectivity of Sulfonated Cation-Exchange Resin for Metal Cations... 

Selectivity of Sulfonated Cation-Exchange Resin for Metal Cations Table 5.5. Retention factors (k) of various cations when sodium perchlorate eluents are used. 

The utility of Nafion far exceeds application as a membrane material. Yeager59 studied the selectivity of Nafion ion-exchange resins toward mono- and divalent cations. The equilibrium constants X(M+/H+) increase continually with hydrated radius, as was previously found for the polystyrene-sulfonate cation exchangers39,40. The differences between the K values enable a facile chromatographic separation of alkali-metal ions, as seen in Figure 9. 

Ion exchange is widely used for the selective separa-tion/removal of metal ions from an aqueous medium either for environmental clean-up, for the removal of undesirable components, or for metal recovery. Other species that have been treated include nitrate, ammonia, and silicate. This process is normally based on the use of polymeric resins (typically styrene/divinylbenzene polymer backbones) that have chemically active groups attached to them. Sulfonic or carboxylate groups are regularly used for the removal of cations, and quaternary ammonium groups are used for the removal of anions. 

Ag and Co functionalized adsorbents for the PPhs adsorption. These transition-metal functionalized adsorbents were prepared by immobilizing Ag and Co onto a solid carrier, for which Amberlyst IS has been selected. Amberlyst 15, a macroreticular polystyrene - crosslinked by divinylbenzene - sulfonated cation exchange resin, has been selected as carrier because of its large pore diameter of approximately 100 [nm]. These macropores ensure the accessibility for the relatively large PPh3 ligands. 

MN-500 over conventional resins in the resistance to oxidation (Table 16.6). Whereas treatment of gel-type exchangers with oxidative solutions results in complete dissolution of resins in 3 days, MN-500 adds only 3% of moisture. The high selectivity of MN-500 with respect to trace metal cations in combination with its high oxidative resistance makes this commercial sulfonate very useful for condensate pohshing. 

Later research has reafHrmed these early results [23]. A separation of alkali metal ions was first attempted in water alone using the lightly sulfonated macro-porous cation exchanger with aqueous 3 mM methanesulfonic acid as the eluent. Under these conditions the sample cations exhibited very similar retention times. The selectivity of the macroporous resin for alkali metal ions was improved con-... 

These results indicate that solvation of the resin plays a role in imparting selectivity for the various sample ions. Microporous cation-exchange resins form a gel and are highly hydrated within. With sulfonated macroporous resins the hydrated aUcah metal ions may be repelled somewhat by the hydrophobic resin matrix. The presence of hydroxymethyl groups on the macroporous resin makes it less hydro-phobic and improves selectivity for the hydrated alkah metal cations. When the sulfonated macroporous resin was used with the same acidic eluent in 100% methanol instead of water, a very good chromatographic separation was obtained. The aUcah metal ions are solvated more with methanol than with water and the resin matrix is probably coated with a thin layer of methanol, which make the ions and resin surface more compatible with one another. 

The tetraethyl ester of 1,1-vinylidenediphosphonic acid has been used to make cross-linked copolymers which are ion-exchange resins with selective chelation properties for toxic metal cations (78). An alternative method for introducing the diphosphonic acid structure is by reaction of a methylenediphosphonic ester with chloromethylated styrene copolymer beads (79). At least one such resin class, Diphonix, also containing sulfonic acid and other functional groups, has shown promise for treatment of radioactive waste and for iron control in copper electrowinning (80,81). 

If selectivity were governed solely by interactions of a purely electrostatic (coulombic) nature one could anticipate negative enthalpies and entropies of exchange due to dominant contributions by A and A5ii respectively. This is indeed found for alkali metal cation exchange on styrenic sulfonate resins as illustrated by Table 5.3a. 

---